ARE ANY 10-VALENCE-ELECTRON HXY SPECIES BENT IN THE GROUND-STATE - ABINITIO OPTIMIZED ENERGIES AND SHAPES OF HBO, HBS, HALO, HALS, AND HCS+ AND THEIR ISOMERS

The potential energy surfaces of the HBO--HOB, HBS--HSB, HA1O--HOA1, HA1S--HSA1, and HCS+--HSC+ systems, each containing ten valence electrons, were exposed by ab initio molecular orbital calculations. Since Walsh's rules predict that all of the energy minima should be linear, and since no nonlinear forms of these molecules or ions have been detected experimentally, it is interesting that theoretical calculations reveal some of them to be distinctly bent in their ground state. The five systems were studied at a minimum of nine uniform levels of theory with basis sets as large as 6-311G(2d,2p), i.e., triple-zeta plus double polarization, and with incorporation of frozen-core and full-core electron correlation at the perturbation orders MP2, MP3, and MP4. The geometry of each stationary-point structure was fully optimized at all levels. At our "best" level of theory, i.e., MP2/6-311G(2d,2p), HSB, HSA1, and HSC+ are strongly bent, HOB is more weakly bent, HOA1 is minutely bent, while HCS+, HBO, HA1O, and HA1S are all linear. All but HSC+ lie in a deep energy well with respect to intramolecular HXY-HYX isomerization. We predict that HSB, HSA1, and HOB should exist as bent molecules under sufficiently isolated conditions, and, if one of these molecules is experimentally realized, it could become the first observed nonlinear ten-valence-electron HXY-type species.